Other forms: wagglier; waggliest
Anything waggly moves rapidly back and forth, like a puppy's waggly tail.
Waggly things waggle, moving up and down or back and forth (or both) in a slightly jerky way. The verb came first, from wag and its Old English root wagian, "move backwards and forwards." You can think of waggly things as wagging in a wiggly way, like a group of children dancing or a roomful of puppies that are very excited to see you.